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					<h1>CodeIgniter User Guide Version 2.0.3</h1>
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		<h1>Database Caching Class</h1>

		<p>The Database Caching Class permits you to cache your queries as
			text files for reduced database load.</p>

		<p class="important">
			<strong>Important:</strong>&nbsp; This class is initialized
			automatically by the database driver when caching is enabled. Do NOT
			load this class manually.<br /> <br /> <strong>Also note:</strong>&nbsp;
			Not all query result functions are available when you use caching.
			Please read this page carefully.
		</p>

		<h2>Enabling Caching</h2>

		<p>Caching is enabled in three steps:</p>

		<ul>
			<li>Create a writable directory on your server where the cache
				files can be stored.</li>
			<li>Set the path to your cache folder in your <dfn>application/config/database.php</dfn>
				file.
			</li>
			<li>Enable the caching feature, either globally by setting the
				preference in your <dfn>application/config/database.php</dfn> file,
				or manually as described below.
			</li>
		</ul>

		<p>Once enabled, caching will happen automatically whenever a page
			is loaded that contains database queries.</p>


		<h2>How Does Caching Work?</h2>

		<p>CodeIgniter's query caching system happens dynamically when
			your pages are viewed. When caching is enabled, the first time a web
			page is loaded, the query result object will be serialized and stored
			in a text file on your server. The next time the page is loaded the
			cache file will be used instead of accessing your database. Your
			database usage can effectively be reduced to zero for any pages that
			have been cached.</p>

		<p>
			Only
			<dfn>read-type</dfn>
			(SELECT) queries can be cached, since these are the only type of
			queries that produce a result.
			<dfn>Write-type</dfn>
			(INSERT, UPDATE, etc.) queries, since they don't generate a result,
			will not be cached by the system.
		</p>

		<p>Cache files DO NOT expire. Any queries that have been cached
			will remain cached until you delete them. The caching system permits
			you clear caches associated with individual pages, or you can delete
			the entire collection of cache files. Typically you'll want to use
			the housekeeping functions described below to delete cache files
			after certain events take place, like when you've added new
			information to your database.</p>

		<h2>Will Caching Improve Your Site's Performance?</h2>

		<p>Getting a performance gain as a result of caching depends on
			many factors. If you have a highly optimized database under very
			little load, you probably won't see a performance boost. If your
			database is under heavy use you probably will see an improved
			response, assuming your file-system is not overly taxed. Remember
			that caching simply changes how your information is retrieved,
			shifting it from being a database operation to a file-system one.</p>

		<p>In some clustered server environments, for example, caching may
			be detrimental since file-system operations are so intense. On single
			servers in shared environments, caching will probably be beneficial.
			Unfortunately there is no single answer to the question of whether
			you should cache your database. It really depends on your situation.</p>

		<h2>How are Cache Files Stored?</h2>

		<p>CodeIgniter places the result of EACH query into its own cache
			file. Sets of cache files are further organized into sub-folders
			corresponding to your controller functions. To be precise, the
			sub-folders are named identically to the first two segments of your
			URI (the controller class name and function name).</p>

		<p>
			For example, let's say you have a controller called
			<dfn>blog</dfn>
			with a function called
			<dfn>comments</dfn>
			that contains three queries. The caching system will create a cache
			folder called
			<kbd>blog+comments</kbd>
			, into which it will write three cache files.
		</p>

		<p>If you use dynamic queries that change based on information in
			your URI (when using pagination, for example), each instance of the
			query will produce its own cache file. It's possible, therefore, to
			end up with many times more cache files than you have queries.</p>


		<h2>Managing your Cache Files</h2>

		<p>Since cache files do not expire, you'll need to build deletion
			routines into your application. For example, let's say you have a
			blog that allows user commenting. Whenever a new comment is submitted
			you'll want to delete the cache files associated with the controller
			function that serves up your comments. You'll find two delete
			functions described below that help you clear data.</p>


		<h2>Not All Database Functions Work with Caching</h2>

		<p>Lastly, we need to point out that the result object that is
			cached is a simplified version of the full result object. For that
			reason, some of the query result functions are not available for use.</p>

		<p>
			The following functions
			<kbd>ARE NOT</kbd>
			available when using a cached result object:
		</p>

		<ul>
			<li>num_fields()</li>
			<li>field_names()</li>
			<li>field_data()</li>
			<li>free_result()</li>
		</ul>

		<p>Also, the two database resources (result_id and conn_id) are
			not available when caching, since result resources only pertain to
			run-time operations.</p>


		<br />

		<h1>Function Reference</h1>



		<h2>$this->db->cache_on()&nbsp; / &nbsp; $this->db->cache_off()</h2>

		<p>Manually enables/disables caching. This can be useful if you
			want to keep certain queries from being cached. Example:</p>

		<code>
			// Turn caching on<br /> $this->db->cache_on();<br /> $query =
			$this->db->query("SELECT * FROM mytable");<br /> <br /> // Turn
			caching off for this one query<br /> $this->db->cache_off();<br />
			$query = $this->db->query("SELECT * FROM members WHERE member_id =
			'$current_user'");<br /> <br /> // Turn caching back on<br />
			$this->db->cache_on();<br /> $query = $this->db->query("SELECT *
			FROM another_table");
		</code>


		<h2>$this->db->cache_delete()</h2>

		<p>Deletes the cache files associated with a particular page. This
			is useful if you need to clear caching after you update your
			database.</p>

		<p>
			The caching system saves your cache files to folders that correspond
			to the URI of the page you are viewing. For example, if you are
			viewing a page at
			<dfn>example.com/index.php/blog/comments</dfn>
			, the caching system will put all cache files associated with it in a
			folder called
			<dfn>blog+comments</dfn>
			. To delete those particular cache files you will use:
		</p>

		<code>$this->db->cache_delete('blog', 'comments');</code>

		<p>If you do not use any parameters the current URI will be used
			when determining what should be cleared.</p>


		<h2>$this->db->cache_delete_all()</h2>

		<p>Clears all existing cache files. Example:</p>

		<code>$this->db->cache_delete_all();</code>
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